Base station transmission control method, cellular system and base station

ABSTRACT

When performing base station selection type transmission power control, transmission of the dedicated physical control channel signal by non-transmitting base stations is halted while minimizing the impact on uplink transmission power control. This decreases downlink interference and increases downlink channel capacity. At a base station, reception quality measuring unit  506  compares the measured uplink reception quality with a target quality, forms a TPC signal on the basis of the results of this comparison, and communicates the contents of this TPC signal to transmission power controller  507 . When the base station in question is a non-transmitting base station, transmission power controller  507  transmits the dedicated physical control channel signal only if the contents of the communicated TPC signal instruct a decrease, and otherwise halts transmission of the dedicated physical control channel signal.

This is a continuation of application Ser. No. 10/066,766 filed Feb. 6,2002, which claims priority from Japanese Patent Application No.036962/2001, filed Feb. 14, 2006, the entire disclosures of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a base station transmission control method, acellular system and a base station. It relates in particular to a basestation transmission control method, and to a cellular system and a basestation, for controlling the transmission of base station controlsignals during handover in a cellular system.

2. Description of Related Art

A cellular system employing direct sequence code division multipleaccess (DS-CDMA) uses the same frequency band in a plurality ofchannels. As a result, from the point of view of a given channel, radiowaves in other channels constitute interference, and an increase in thepower of this interference can result in a deterioration of receptionquality for the desired signal, and in the connection being dropped,etc. Hence the number of channels through which communication can besustained at the required reception quality, in other words, the channelcapacity, is dependent on the amount of interference. In the uplink, asignal transmitted by a mobile station located far from a base stationundergoes more attenuation than a signal transmitted by a mobile stationlocated near that base station, and hence transmitting such signals atthe same power gives rise to the near-far effect in which theinterfering signal becomes stronger than the desired signal andcommunication becomes difficult. Transmission power control is thereforean essential technique for uplink channels, where it involvescontrolling the transmission power of the mobile stations so that theirsignals are received at a base station at the same received power.

Although a near-far effect of this sort does not occur for downlinkchannels, in order to reduce interference to other channels,transmission power control is performed at base stations so thattransmission power is kept at the minimum necessary for mobile stationsto maintain the required reception quality. Control of transmissionpower at the base stations is based on closed-loop control. Thisinvolves comparing the measured reception quality with a prescribedtarget quality, and if it is found to be higher than the target quality,a transmission power control (TPC) signal instructing that transmissionpower be decreased is transmitted, while if the measured receptionquality is found to be lower than the target quality, a TPC signalinstructing that transmission power be increased is transmitted.

Customarily, a technique called soft handover is used in a cellularsystem employing CDMA. Soft handover involves transmitting from aplurality of base stations to which a mobile station is connectedsimultaneously, and is employed when a mobile station approaches thevicinity of a cell boundary and the difference between the propagationloss experienced by the signal received from the base station to whichthe mobile station is connected and the propagation loss experienced bythe signal received from a neighbouring base station falls within aprescribed threshold. Due to the diversity effect obtained bytransmitting from a plurality of base stations, soft handover results inimproved reception quality when a mobile station is near a cellboundary, where propagation loss is large and reception quality tends todeteriorate. Because soft handover sets up a connection to a basestation which is the candidate for the next connection before theconnection to the active base station is released, handover is smoothand without any momentary loss of signal. Because a mobile station isgenerally at a different distance from each of several base stations,each of these base stations sees a different uplink reception qualityduring soft handover. Hence in the IMT-2000 standard, for example,during soft handover each base station notifies the base stationcontroller of the reception quality of the signal obtained by RAKEcombining, whereupon selection combining is performed using the signalwith the highest reception quality as the desired signal. Uplinktransmission power control in the IMT-2000 standard therefore involvescomparing the uplink reception quality at each base station with aprescribed target reception quality, forming TPC signals on the basis ofthese results, and transmitting these TPC signals to the mobile station.If even one of the plurality of TPC signals received at the mobilestation instructs a decrease in transmission power, the mobile stationdecreases its transmission power, while if all the TPC signals instructan increase in transmission power, the mobile station increases itstransmission power. This provides transmission power control thatreduces any excess uplink transmission power while satisfying therequired reception quality.

On the other hand, a problem encountered in the downlink is that becausea plurality of base stations are transmitting during soft handover,there is an increase in interference and hence a decrease in channelcapacity. As a solution to this problem, JP H11-069416 A discloses atechnique for restricting the base stations that transmit during softhandover, which serves to decrease the downlink interference.Specifically, base stations transmit a pilot signal at a prescribedpower, and the mobile station measures the reception quality of thepilot signals transmitted by those base stations that have set up aconnection. The base station with the highest reception quality is takenas the transmitting base station, which is called as “primary cell”,while the other base stations stop transmitting. This ensures that onlybase stations with low propagation loss transmit, with the result thatdownlink interference is decreased and channel capacity can be expectedto increase.

The site selection diversity TPC (SSDT) scheme in the IMT-2000 standardapplies this technique. A base station usually transmits both dedicatedphysical control channel signals and dedicated physical data channelsignals in the downlink. In SSDT, base stations other than thetransmitting base station (i.e., non-transmitting base stations) haltthe transmission of dedicated physical data channel signals only, andcontinue to transmit dedicated physical control channel signals (3G TS25.214 3rd Generation Partnership Project; Technical Specification GroupRadio Access Network; Physical layer procedures (FDD) (Release 1999)5.2.1.4.5). The reason for this is that, as mentioned previously, eachbase station that has set up a connection to the mobile station has itsown TPC signal and hence even the non-transmitting base stations have touse the dedicated physical control channel to transmit these TPCsignals.

However, the proportion of dedicated physical control channel signalsand dedicated physical data channel signals transmitted in the downlinkvaries according to transmission rate and other factors. For example, atlow transmission rates, the proportion of dedicated physical controlchannel signals becomes higher than that of dedicated physical datachannel signals. An excessively high proportion of dedicated physicalcontrol channel signals results in the problem that the dedicatedphysical control channel signals transmitted by non-transmitting basestations will have a significant interference impact on other channels,resulting in decreased channel capacity.

It may be pointed out that not transmitting any downlink dedicatedphysical control channel signals at all from the non-transmitting basestations would be tantamount to not transmitting any TPC signals forcontrolling transmission power in the uplink, which would give rise tothe problem that uplink reception quality might deteriorate.

SUMMARY OF THE INVENTION

The present invention therefore addresses itself to the task ofdecreasing the interference resulting from the dedicated physicalcontrol channel signals transmitted by non-transmitting base stations inthe downlink while minimizing the effect of this on the uplink.

To achieve this object, the present invention provides a base stationtransmission control method wherein;

a mobile station sets up a connection to one or more base stations,measures the reception quality of the pilot signal transmittedtherefrom, and in accordance with the results of this measurement,designates one or more of the base stations with which a connection hasbeen set up (hereinafter termed “connected base stations”) as atransmitting base station, and sends notification of this to theconnected base stations;

when a connected base station has been designated as a transmitting basestation, it transmits dedicated physical control channel signals anddedicated physical data channel signals to the aforementioned mobilestation, measures the uplink reception quality, and transmits,multiplexed in the dedicated physical control channel signal, atransmission power control signal serving to control the transmissionpower of the mobile station so that the aforementioned reception qualityapproaches a prescribed target quality;

the mobile station receives one or more dedicated physical controlchannel signals transmitted from connected base stations and controlsits own transmission power in accordance therewith; and

when a connected base station has been designated as a non-transmittingbase station, it halts transmission of the dedicated physical datachannel signal and decides whether or not to transmit the dedicatedphysical control channel signal in accordance with the uplink receptionquality.

In this base station transmission control method, it is also feasiblefor non-transmitting base stations to perform control in accordance withthe rate of movement of the mobile station. Namely, when the movement ofthe mobile station is faster than a prescribed threshold, thetransmission of dedicated physical control channel signals and dedicatedphysical data channel signals at non-transmitting base stations can behalted; and when the movement of the mobile station is slower than theprescribed threshold, the transmission of dedicated physical datachannel signals from non-transmitting base stations can be halted andwhether or not to transmit the dedicated physical control channel signalcan be decided in accordance with the uplink reception quality.

In the cellular system of this invention;

a mobile station sets up a connection to one or more base stations,measures the reception quality of the pilot signal transmittedtherefrom, and in accordance with the results of this measurement,designates one or more of the connected base stations as a transmittingbase station, and sends notification of this to the connected basestations;

when a connected base station has been designated as a transmitting basestation, it transmits dedicated physical control channel signals anddedicated physical data channel signals to the aforementioned mobilestation, measures the uplink reception quality, and transmits,multiplexed in the dedicated physical control channel signal, atransmission power control signal serving to control the transmissionpower of the mobile station so that the aforementioned reception qualityapproaches a prescribed target quality;

the mobile station receives one or more dedicated physical controlchannel signals transmitted from connected base stations and controlsits own transmission power in accordance therewith; and

when a connected base station has been designated as a non-transmittingbase station, it halts transmission of the dedicated physical datachannel signal and decides whether or not to transmit the dedicatedphysical control channel signal in accordance with the uplink receptionquality.

In this cellular system of the invention, it is also feasible fornon-transmitting base stations to perform control in accordance with therate of movement of the mobile station. Namely, when the movement of themobile station is faster than a prescribed threshold, the transmissionof dedicated physical control channel signals and dedicated physicaldata channel signals at non-transmitting base stations can be halted;and when the movement of the mobile station is slower than theprescribed threshold, the transmission of dedicated physical datachannel signals from non-transmitting base stations can be halted andwhether or not to transmit the dedicated physical control channel signalcan be decided in accordance with the uplink reception quality.

A base station according to the present invention;

transmits a pilot signal at a prescribed power;

receives information relating to transmitting base stations from amobile station which has set up a connection to one or more basestations, measured the reception quality of the pilot signal transmittedtherefrom, and in accordance with the results of this measurement,designated one or more of the connected base stations as a transmittingbase station, and sent notification of this to the connected basestations;

when the base station in question is a transmitting base station, ittransmits dedicated physical control channel signals and dedicatedphysical data channel signals; and

when the base station in question is a non-transmitting base station, ithalts transmission of the dedicated physical data channel signal anddecides whether or not to transmit the dedicated physical controlchannel signal in accordance with the uplink reception quality.

Alternatively, when the estimated rate of movement of the mobile stationis faster than the prescribed threshold, this base station of theinvention stops transmitting irrespective of uplink reception quality.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings inwhich;

FIG. 1 shows an example of downlink and uplink receiving levelfluctuations when the rate of movement of a mobile station is slow, andaids an explanation of the principles behind embodiments of thisinvention;

FIG. 2 shows an example of downlink and uplink receiving levelfluctuations when the rate of movement of a mobile station is fast, andaids an explanation of the principles behind embodiments of thisinvention;

FIG. 3 shows, as a function of the rate of movement of a mobile station,the probability of a non-transmitting base station having the highestuplink receiving level, and aids an explanation of the principles behindembodiments of this invention;

FIG. 4 shows an exemplary configuration of the cellular system of thepresent invention;

FIG. 5 is a function block diagram of a base station according to afirst mode of embodying the invention;

FIG. 6 is a flowchart showing the operation of a base station accordingto a first mode of embodying the invention;

FIG. 7 is a function block diagram of a base station according to asecond mode of embodying the invention;

FIG. 8 is a flowchart showing the operation of a base station accordingto a second mode of embodying the invention;

FIG. 9 is a function block diagram of a base station according to athird mode of embodying the invention; and

FIG. 10 is a flowchart showing the operation of a base station accordingto a third mode of embodying the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the DS-CDMA based cellular system of the present invention, a mobilestation sets up a connection to one or more base stations, measures thereception quality of a pilot signal transmitted therefrom, and inaccordance with the results of this measurement, designates one or moreof the base stations with which a connection has been set up(hereinafter termed “connected base stations”) as a transmitting basestation, and sends notification of this to the connected base stations.When a connected base station is, according to the notification from themobile station, a transmitting base station, it transmits dedicatedphysical control channel signals and dedicated physical data channelsignals to the mobile station. When a connected base station is anon-transmitting base station, it halts transmission of the dedicatedphysical data channel signal and decides whether or not to transmit thededicated physical control channel signal in accordance with thevelocity of the mobile station and the uplink reception quality.

A base station measures the uplink reception quality and transmits,multiplexed onto the downlink, a transmission power control signal forcontrolling the transmission power of the mobile station.

The mobile station receives one or more transmission power controlsignals and controls its own transmission power. Because there is adifferent propagation loss between the mobile station and each connectedbase station, there is also a different uplink reception quality at eachbase station. The contents of the transmission power control signal forcontrolling mobile station transmission power are therefore different ateach base station.

In order to make transmission power in the uplink as small as possibleso as to minimize interference with other users, it is preferable totransmit at a transmission power that satisfies the prescribed qualityrequirements at the base station with the smallest propagation loss.Hence uplink interference can be decreased by transmitting in conformitywith the transmission power control signal from the base station atwhich uplink propagation loss is smallest.

On the other hand, a “transmitting base station” is a base station forwhich downlink propagation loss is small. Because there is usually nocorrelation between fading in the downlink and the uplink, a basestation for which downlink propagation loss is minimum is notnecessarily the base station at which uplink propagation loss isminimum. However, because shadowing and distance-related attenuation,which account for a large proportion of propagation loss, are the samein the uplink and the downlink, a base station for which downlinkpropagation loss is small has a high probability of experiencing a smallpropagation loss in the uplink as well.

Hence for uplink transmission power control, the transmission powercontrol signal transmitted from a transmitting base station isimportant, whereas the transmission power control signal transmittedfrom a non-transmitting base station is of little importance. It followsthat stopping transmission power control signals from non-transmittingbase stations has little impact on the uplink and serves to reducedownlink interference.

As was mentioned in connection with the related art, in order to keepinterference in the uplink to a minimum, transmission power control at amobile station that has set up connections with a plurality of basestations decreases the transmission power if even a single TPC signalinstructing transmission power to be decreased is received. It followsthat if TPC signals instructing an increase in transmission power arenot transmitted from non-transmitting base stations, the transmissionpower control performed at the mobile station is the same as if all theTPC signals were transmitted.

Accordingly, in the present invention a non-transmitting base stationtransmits the dedicated physical control channel signal only when itwill be transmitting a TPC signal instructing a decrease, in otherwords, only when the uplink reception quality is higher than a targetquality. Otherwise, the non-transmitting base station halts transmissionof dedicated physical control channel signals. This technique has noimpact on the uplink, and serves to decrease downlink interference andto increase downlink channel capacity.

It may be mentioned that the probability of a non-transmitting basestation transmitting a TPC signal instructing a decrease differsaccording to the rate of movement of the mobile station. The reason forthis is explained below.

A non-transmitting base station is a base station associated with a highpropagation loss in the downlink. However, fading in the downlink is notcorrelated with fading in the uplink.

When mobile station velocity is low, as shown in FIG. 1, there will beoccasions (indicated by an arrow) when a non-transmitting base station(base station B) experiences the minimum propagation loss and thereforethe maximum receiving level in the uplink.

On the other hand, when mobile station velocity is high, as shown inFIG. 2, because fading is averaged within the measurement interval, theprobability of a non-transmitting base station being the base stationwith the highest uplink receiving level is very small.

Consequently, as shown in FIG. 3, the probability of a non-transmittingbase station having the highest uplink receiving level decreases withincreasing rate of movement of the mobile station.

Because transmission power control at a mobile station operates so as tomaintain the minimum reception quality required at the base station withthe highest receiving level, TPC signals instructing a decrease arenaturally transmitted by the base station experiencing the highestuplink receiving level.

It follows that when the rate of movement of a mobile station is high,the probability of a non-transmitting base station transmitting a TPCsignal instructing a decrease is small. Moreover, when the rate ofmovement of a mobile station is high, the ability of TPC signals tofollow fading deteriorates and hence transmission power control itselfis less effective. Consequently, when a mobile station is movingquickly, transmitting no TPC signals at all from non-transmitting basestations has little impact on the uplink.

Accordingly, in the present invention, when a mobile station is movingquickly, downlink interference can be further decreased by alwayshalting the transmission of dedicated physical control channel signalsby a non-transmitting base station.

In FIG. 4, which shows an exemplary configuration of the cellular systemof the present invention, base stations 411 to 413 transmit to mobilestations 421 and 422 located within cells 401 to 403. Each base stationtransmits, to all mobile stations within the cell, a common pilot signalof prescribed power. Each base station also transmits, to each mobilestation within the cell, dedicated physical control channel signals anddedicated physical data channel signals. The transmission power of thesesignals is controlled by fast closed-loop transmission power control.

The mobile stations set up a connection to the base station whose commonpilot signal has been received at the highest receiving level, and toone or more base stations whose common pilot signal has been received ata receiving level that differs from this highest receiving level by amargin that is within a prescribed threshold.

Mobile station 421 located near the centre of cell 401 sets up aconnection to base station 411 and this base station alone when thefollowing two conditions obtain: namely, when the receiving level of thecommon pilot signal transmitted by base station 411 is highest and whenthe difference between this receiving level and the receiving levels ofthe common pilot signals transmitted by the other base stations is notwithin the prescribed threshold.

Mobile station 422 is located near the boundaries of cells 401, 402 and403, and sets up connections simultaneously to base stations 411, 412and 413 if the differences in the receiving level of the common pilotsignal transmitted by these three base stations are within a prescribedthreshold.

If base station 412 is not within the threshold that determines atransmitting base station, only base stations 411 and 413 transmitdedicated physical control channel signals and dedicated physical datachannel signals. In this case, base station 412 halts transmission ofthe dedicated physical data channel signal, and decides whether or notto transmit the dedicated physical control channel signal in accordancewith the uplink reception quality.

Various modes of embodying the present invention will now be describedwith reference to the accompanying drawings.

FIRST EMBODIMENT

In FIG. 5, which is a block diagram of a base station according to afirst mode of embodying this invention, the base station comprisesantenna 501 for receiving signals transmitted from mobile stations,duplexer 502, radio receiver 503 for converting radio signals toreceived baseband signals, RAKE receiver 504 for combining basebandsignals, demultiplexer 505, reception quality measuring unit 506 formeasuring the reception quality of the RAKE-combined received signal,transmission power controller 507 for controlling the transmission powerof the base station, multiplexer 508, spreading circuit 509 forspreading the downlink signal and outputting a baseband signal fortransmission, and radio transmitter 510 for converting this basebandsignal to a radio signal and transmitting it.

Demultiplexer 505 separates the signal that specifies the transmittingbase stations, this signal being transmitted from the mobile station atprescribed intervals, and notifies transmission power controller 507 ofwhether or not the local base station is a transmitting base station.

Reception quality measuring unit 506 compares the measured uplinkreception quality with the target quality. If the measured receptionquality is lower than the target quality, it forms a TPC signalinstructing an increase, while if the measured reception quality ishigher, it forms a TPC signal instructing a decrease. The TPC signal ismultiplexed in the dedicated physical control channel signal bymultiplexer 508.

The contents of the TPC signal formed by reception quality measuringunit 506 are communicated to transmission power controller 507. If thelocal base station is a non-transmitting base station, transmissionpower controller 507 decides whether or not to transmit the dedicatedphysical control channel signal in accordance with the contents of thecommunicated TPC signal.

FIG. 6 is a flowchart showing the operation of transmitting dedicatedphysical control channel signals and dedicated physical data channelsignals from a base station according to a first mode of embodying theinvention. When it is time to update the transmitting base station, thebase station receives a signal that specifies the transmitting basestations, this signal being transmitted from the mobile station (Step602). If measurement of the uplink reception quality SIRrec andcomparison with the target quality SIRtrg shows that SIRrec<SIRtrg, aTPC signal instructing an increase is formed, while if the measurementand comparison shows that SIRrec>SIRtrg, a TPC signal instructing adecrease is formed (Step 603). If the local base station is atransmitting base station (Step 604, YES), it transmits the dedicatedphysical data channel signal and the dedicated physical control channelsignal in which the TPC signal has been multiplexed (Step 605). If thelocal base station is a non-transmitting base station (Step 604, NO), ithalts transmission of the dedicated physical data channel signal, buttransmits the dedicated physical control channel signal in which the TPCsignal has been multiplexed if the TPC signal instructs a decrease (Step607). On the other hand, if the TPC signal instructs an increase, the(non-transmitting) base station stops transmitting (Step 608). The basestation performs the foregoing control sequence at prescribed TPCcontrol intervals.

As has been described above, in this mode of embodying the invention, anon-transmitting base station decides whether or not to transmit thededicated physical control channel signal in accordance with thecontents of the TPC signal for controlling uplink transmission power,this TPC signal being transmitted after being multiplexed in thedownlink dedicated physical control channel signal. The transmissionpower control performed by the mobile station is such that it decreasesits transmission power if it receives even a single TPC signalinstructing a decrease. It follows that if non-transmitting basestations transmit only when the TPC signal instructs a decrease, themobile station can perform the same transmission power control as whenall the TPC signals are transmitted. Hence transmission of the dedicatedphysical control channel signals that non-transmitting base stationstransmit on the downlink can be stopped without affecting TPC-basedtransmission power control in the uplink. As a result, downlinkinterference can be reduced and channel capacity increased.

SECOND EMBODIMENT

In FIG. 7, which is a block diagram of a base station according to asecond mode of embodying this invention, parts equivalent to parts inFIG. 5 have the same referencing numerals and will not be describedagain. The cellular system to which this embodiment is applied is thesame as that depicted in FIG. 4.

A base station according to this second embodiment of the inventionillustrated in FIG. 7 has velocity information extractor 711, which thebase station shown in FIG. 5 did not have. In the first embodiment, whena TPC signal instructing a decrease is to be transmitted from anon-transmitting base station, that non-transmitting base stationinvariably transmitted the dedicated physical control channel signal,regardless of the rate of movement of the mobile station. However, inthis second embodiment, the control performed by a non-transmitting basestation is changed in accordance with the estimated velocity of themobile station. Hence velocity information extractor 711 is provided toextract velocity information from the mobile station and communicate theinformation to the transmission power controller. This velocityinformation can be arranged at a prescribed location in the signalframe, or it can be carried in the signal frame together with a flagindicating that it is velocity information. The velocity information canbe transmitted at prescribed intervals.

When the estimated velocity of the mobile station is higher than aprescribed threshold, transmission power controller 507 haltstransmission by a non-transmitting base station regardless of thecontents of the TPC signal. When the estimated velocity is less than theprescribed threshold, transmission power controller 507 performs thesame operations as in the first embodiment.

The operation of the other constituent elements of the embodimentillustrated in FIG. 7 is the same as that of the base station shown inFIG. 5.

FIG. 8 is a flowchart showing the operation of transmitting dedicatedphysical control channel signals and dedicated physical data channelsignals from a base station according to a second mode of embodying theinvention. The base station receives a velocity information signal fromthe mobile station, this signal being transmitted at prescribedintervals (Step 802). When it is time to update the transmitting basestations, the base station receives a signal that specifies thetransmitting base stations, this signal being transmitted from themobile station (Step 804). If measurement of the uplink receptionquality SIRrec and comparison with the target quality SIRtrg shows thatSIRrec<SIRtrg, a TPC signal instructing an increase is formed, while ifthe measurement and comparison shows that SIRrec>SIRtrg, a TPC signalinstructing a decrease is formed (Step 805). If the local base stationis a transmitting base station (Step 806, YES), it transmits thededicated physical data channel signal and the dedicated physicalcontrol channel signal in which the TPC signal has been multiplexed(Step 807). If the local base station is a non-transmitting base station(Step 806, NO), the extracted estimated velocity is compared with aprescribed threshold (Step 808). If it is found to be higher than theprescribed threshold, transmission of both the dedicated physical datachannel signal and the dedicated physical control channel signal ishalted (Step 809). On the other hand, if the estimated velocity is lowerthan the prescribed threshold, transmission of the dedicated physicaldata channel signal is halted and the dedicated physical control channelsignal is transmitted only if the TPC signal instructs a decrease (Step811), otherwise, transmission of the dedicated physical control channelsignal is halted as well (Step 812).

The base station performs the foregoing control sequence repeatedly atprescribed TPC control intervals.

Thus according to this mode of embodying the invention, transmission ofdedicated physical control channel signals by a non-transmitting basestation is invariably halted if the rate of movement of the mobilestation is fast. If the rate of movement is fast, the probability of abase station at which uplink propagation loss is minimum also being thebase station at which downlink propagation loss is minimum, is veryhigh. As described above, it follows that the probability of anon-transmitting base station transmitting a TPC signal instructing adecrease becomes very small at high mobile station movement rates.Because the mobile station decreases its transmission power if there isat least one TPC signal that instructs a decrease, ceasing to transmitTPC signals instructing an increase from non-transmitting base stationshas no impact on TPC control. Moreover, when the rate of movement of amobile station is high, TPC control is unable to follow fading and henceTPC control itself has little effect. Consequently, when a mobilestation is moving very quickly, completely halting the transmission ofdedicated physical control channel signals from non-transmitting basestations has little impact on TPC control in the uplink and provides asignificant decrease in downlink interference. When the rate of movementof a mobile station is slow, the non-transmitting base stations transmitonly TPC signals that instruct a decrease. This results in uplinkcontrol which is the same as if all TPC signals were transmitted, andalso provides the maximum possible decrease in downlink interference.

THIRD EMBODIMENT

In FIG. 9, which shows the configuration of a base station according tothird mode of embodying this invention, parts equivalent to parts inFIG. 5 have the same referencing numerals and will not be describedagain. The cellular system to which this embodiment is applied is thesame as that depicted in FIG. 4.

A base station according to this third embodiment of the inventionillustrated in FIG. 9 has velocity estimator 911, which the base stationshown in FIG. 5 did not have. In the second embodiment, a velocityinformation signal transmitted by the mobile station was extracted andthe control of uplink transmission power performed by a non-transmittingbase station was made dependent on this extracted velocity informationsignal. However, in this third embodiment, the rate of movement of themobile station is estimated by the base station. Hence velocityestimator 911 is provided in the base station to estimate the rate ofmovement of the mobile station from signal fading, for example, fromchanges in the TPC signal that is transmitted at prescribed intervalsfrom the mobile station to control the transmission power of the basestation. This estimated rate of mobile station movement is communicatedto the transmission power controller. The operation of the otherconstituent elements of the embodiment illustrated in FIG. 9 is the sameas that of the base station shown in FIG. 7.

FIG. 10 is a flowchart showing the operation of transmitting dedicatedphysical control channel signals and dedicated physical data channelsignals from a base station according to this third embodiment of theinvention. Those portions of the operation that are equivalent toportions in FIG. 8 have the same referencing numerals and will not bedescribed again. In this third embodiment the base station does notreceive a velocity information signal at prescribed intervals, as in thesecond embodiment whose operation is shown in FIG. 8. Instead, the basestation estimates the velocity of the mobile station from changes in theTPC signal that is transmitted from the mobile station at prescribedintervals (Step 1002). The rest of the operation depicted in FIG. 10 isthe same as the base station operation shown in FIG. 8.

This third mode of embodying the invention is similar to the second inthat the transmission of dedicated physical control channel signals by anon-transmitting base station is invariably halted if the rate ofmovement of the mobile station is fast. If the rate of movement is fast,the probability of a base station at which uplink propagation loss isminimum also being the base station at which downlink propagation lossis minimum, is very high. As described above, it follows that theprobability of a non-transmitting base station transmitting a TPC signalinstructing a decrease becomes very small at high mobile stationmovement rates. Because the mobile station decreases its transmissionpower if there is at least one TPC signal that instructs a decrease,ceasing to transmit TPC signals instructing an increase fromnon-transmitting base stations has no impact on TPC control. Moreover,when the rate of movement of a mobile station is high, TPC control isunable to follow fading and hence TPC control itself has little effect.Consequently, when a mobile station is moving very quickly, completelyhalting the transmission of dedicated physical control channel signalsfrom non-transmitting base stations has little impact on TPC control inthe uplink and provides a significant decrease in downlink interference.When the rate of movement of a mobile station is slow, thenon-transmitting base stations transmit only TPC signals that instruct adecrease. This results in uplink control which is the same as if all TPCsignals were transmitted, and also provides the maximum possibledecrease in downlink interference.

The present invention is not restricted to the foregoing embodiments.

For example, the method of estimating the rate of movement of the mobilestation in this invention is not restricted to the methods of theembodiments, and it is feasible to use an estimate of mobile stationvelocity that is obtained by some other means.

Moreover, although a cellular system based on DS-CDMA was used in theforegoing embodiments, the invention is not restricted to this and otheraccess schemes can be used.

As has been explained above, the present invention halts thetransmission of dedicated physical control channel signals bynon-transmitting base stations in accordance with uplink receptionquality or in accordance with the content of the transmission powercontrol signal to be transmitted in the downlink. The inventiontherefore does not affect transmission power control in the uplink, andcan decrease interference due to the dedicated physical control channelsignals that non-transmitting base stations transmit in the downlink. Asa result, downlink channel capacity can be increased.

Furthermore, according to the invention, transmission of dedicatedphysical control channel signals by non-transmitting base stations isinvariably halted when the velocity of a mobile station is very high,and completely ceasing to transmit transmission power control signalsfrom non-transmitting base stations has little impact on transmissionpower control in the uplink. The invention can therefore furtherdecrease downlink interference and greatly increase channel capacity.

1. A method for controlling transmission power of a mobile stationconnecting with a plurality of base stations, the method comprising:transmitting, in a specified base station among the plurality of basestations, a transmission power control signal for the mobile station;and transmitting in any of the other base stations, a transmission powercontrol signal for the mobile station, wherein the specified basestation is allowed to transmit a transmission, power control signal toincrease transmission power, and any of the other base station is notallowed to transmit a transmission power control signal for the mobilestation to increase transmission power but is allowed to transmit atransmission power control signal for the mobile station to decreasetransmission power.
 2. A method for controlling transmission power froma base station to a mobile station, the method comprising; transmittinga transmission power control signal for the mobile station, wherein thebase station is allowed to transmit a transmission power control signalfor the mobile station to increase transmission power when the basestation is a transmitting base station of the mobile station, and thebase station is not allowed to transmit a transmission power controlsignal for the mobile station to increase transmission power but isallowed to transmit a transmission power control signal for the mobilestation to decrease transmission power when the base station is anon-transmitting base station of the mobile station; and receiving thetransmission power control signal from the base station to the mobilestation and controlling the transmission power according to the receivedtransmission power control signal.
 3. A mobile communication systemcomprising a mobile station and a plurality of base stations connectingwith the mobile station, the plurality of base stations consisting of aspecified base stations and the other base stations, wherein thespecified base station transmits a transmission power control signal forthe mobile station; any of the other base stations transmits atransmission power control signal for the mobile station; the specifiedbase station is allowed to transmit a transmission power control signalfor the mobile station to increase transmission power; and any of theother base station is not allowed to transmit a transmission controlsignal for the mobile station to increase transmission power but isallowed to transmit a transmission power control signal for the mobilestation to decrease transmission power.
 4. A mobile communication systemcomprising a mobile station and a base station connecting with themobile station, wherein the base station is allowed to transmit atransmission power control signal for the mobile station to increasetransmission power when the base station is a transmitting base stationof the mobile station; the base station is not allowed to transmit atransmission power control signal for the mobile station to increasetransmission power but is allowed to transmit a transmission powercontrol signal for the mobile station to decrease transmission powerwhen the base station is a non-transmitting base station of the mobilestation; and the mobile station receives the transmission power controlsignal from the base station to the mobile station and controls thetransmission power according to the received transmission power controlsignal.
 5. A base station connecting with a mobile station, wherein thebase station is allowed to transmit a transmission power control signalfor the mobile station to increase transmission power when the basestation is a transmitting base station of the mobile station; the basestation is not allowed to transmit a transmission power control signalfor the mobile station to increase transmission power but is allowed totransmit a transmission power control signal for the mobile station todecrease transmission power when the base station is a non-transmittingbase station of the mobile station; and the mobile station receives thetransmission power control signal from the base station to the mobilestation and controls the transmission power according to the receivedtransmission power control signal.